If you would like to redirect the all HTTP traffic through the proxy without needing to set up a proxy manually in all your applications you will need to add some rules
iptables -t nat -A PREROUTING -i eth1 -p tcp -m tcp --dport 80 -j DNAT --to-destination 192.168.0.1:3128 iptables -t nat -A PREROUTING -i eth0 -p tcp -m tcp --dport 80 -j REDIRECT --to-ports 3128
Where eth1,eth0 are the LAN, WAN devices and 192.168.0.1 is the IP address of your LAN device.
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